Vehicle occupant protection systems have, in prior times, used a distributed array of inertia-operated mechanical switches to sense the onset of a collision and trigger the actuation of an occupant protection device (e.g., an air bag or a seat belt retractor). More recently, the type of system that is typically used in vehicles includes one or more electronic acceleration sensors working in cooperation with a microprocessor and sophisticated software. The sensors provide electronic signals proportional to vehicle acceleration. The microprocessor assesses the frequent and rapid changes in the vehicle's acceleration and determines whether a crash event is in progress that requires the actuation of the occupant protection system.
Protection systems using acceleration sensors typically still include one inertia-operated mechanical switch, usually referred to as a "safing switch", for providing a redundant level of detection of the crash event. The safing switch is designed and calibrated to close at a relatively early stage in a crash event. Closure of the safing switch is a necessary condition but not the sole determinant in controlling actuation of the occupant protection system, however. The occupant protection device (e.g., the air bag) is only actuated when the microprocessor determines that the severity of the crash is sufficient to warrant such actuation.
Safing switches presently in use employ an inertial mass that moves, upon vehicle deceleration, against the resilience of a spring. Electrical contacts associated with the switch are coupled to the mass in such a way that the contacts close when the mass moves a certain distance from its initial, or rest, position. Since the deceleration impulse representative of a significant crash varies from one vehicle type to another, the switch design often must be modified to some extent from one vehicle type to another such that the switch will, in each case, close at the appropriate time for that particular vehicle type. Normal manufacturing tolerances, however, still cause the switch actuation point to vary from switch to switch. Thus, each individual safing switch is calibrated during manufacture to "fine tune" the switch closure to the proper deceleration impulse point selected for that vehicle type.
Safing switches now in use are manufactured by processes generally similar to those used to assemble other small mechanical devices. Efforts have been made, however, to develop smaller switches that could be manufactured using techniques like those used to manufacture semiconductor components and/or micro-machined silicon elements. Patents describing such micro-machined switch devices include U.S. Pat. Nos. 5,177,331, 4,855,544, and 4,543,457.
U.S. Pat. No. 5,177,331 discloses an "impact sensor" formed of micro-machined silicon. The "impact sensor" includes contacts that close when the sensor is subjected to an impact. The sensor further includes electrodes between which an electrostatic field appears, creating a force tending to urge the contacts to close. When the contacts are in their unactuated position, the force is too small to perturb the contacts into the closed position. When the contacts close due to an impact, however, the nearness of the electrodes substantially increases the magnitude of the electrostatic field and the resulting force. The increased electrostatic force latches the contacts in the closed position until the field is released.